Retractable assemblies are widely used in analytical measurements technology. They serve for withdrawing sensors from, and then reintroducing them back into, a process, without process interruption, even in the presence of high process-pressures. The sensor is, in this connection, moved, automatically or manually, from a measuring position into a maintenance- or service-position, and then back into the measuring position. In the maintenance position, the sensor is checked, calibrated or, perhaps, simply cleaned, which, depending on the particular application, can be highly important for the quality of the measurements. In the measuring position, the process variable to be ascertained or monitored is sensed.
At process termination, or process interface, to the medium, two different solutions are in use. In a first form of embodiment, a ball valve, or a plug valve, is used, which, by turning, opens or closes to the process the opening, through which the sensor is moved into, and out of, the process. In a second form of embodiment, a closing plug is used. This closing plug is an integral component of the retractable assembly.
Ball valve assemblies are preferably used in the case of media containing a solids fraction. “Solids” include fibers, and, also, accreted lime and similar materials. The sensor in ball valve assemblies is moved into, and out of, the process separated from the closing-/opening mechanism. In this connection, the ball valve is, respectively, closed and opened. For the purpose of cleaning the sensor, the sensor is brought in the maintenance position into a rinsing chamber separated from the process.
Retractable assemblies are available from Endress+Hauser in different embodiments under the mark ‘CleanFit’. Assemblies with a closing plug are, for example, carried under the identifiers Clean Fit S, CPA 471, CPA 472, CPA 473 and CPA 474. In the case of these retractable assemblies, the holder for the sensor is itself embodied as a sealing element. The front part of the holder is in the form of a plug, which already seals radially during movement back to the process connection. The construction of the sealing system of the assemblies CPA 471 and CPA 472 assures, in such case, a perfect separating of the rinsing chamber and, thus, the “outside world”, and the process—and, indeed, in every position of the holder.
Of course, this separating is only effective, when the sealing rings are not damaged. A diligent checking of the sealing rings is, consequently, highly recommended. However, checking means, in the case of many assemblies, disassembling the complete retractable assembly and is, thus, burdened with considerable consumption of time. Also, it is to be heeded, that the checking, or replacing, of the sealing rings is to be performed only by schooled personnel, since the retractable assembly must then again work correctly under the reigning process conditions.
A replacement of the sealing rings is necessary at certain time intervals for the purpose of assuring the sealing function, since O-rings, which are usually made of polymers, after a certain amount of service time, tend to stick to a surface, against which they are pressed. If, then, the holder moves relative to the assembly housing, there is the danger, that the surface of the polymer, thus the O-ring, will be damaged. This effect is known under the name, “slip-stick effect”. Upon occurrence of the effect, the sealing function of an O-ring is no longer assured.
From the above discussion, it becomes clear, that the number of stroke movements has a relatively large influence on the functioning of the sealing rings and, therewith, on the state of sealing of the retractable assembly. Other influencing variables as regards wear of the sealing rings are temperature and pressure. In the past, these variables have not been taken into consideration in the setting of maintenance intervals. Rather, separation of consecutive maintenance intervals has been based on empirical estimates.